In the atmosphere, cumulus clouds, or low-level clouds, begin to form at what meteorologists call the Lifted Condensation Level (LCL).
The LCL is the level at which lifted or rising water vapor condenses into a cloud droplet, hence the name.
Water will not condense below this level to form a cloud.
Generally speaking, the LCL is uniform as far as the eye can see. Therefore, the LCL determines the bottom level of clouds…thus, the flat look on the bottom of cumulus clouds.